This invention relates, in general, to bubble memories and, in particular, to lattice file bubble memory techniques.
Reference is made to in the article by Cohen and Chang, Proceedings of "IEEE" Volume 63, Number 8, August, 1975, entitled, "The Frontiers of Magnetic Bubble Technology", as an example of the existing state of the art in lattice file memories. In this article, the authors discuss the improvement in the packing density in bubble lattice file configurations over existing bubble memories of the non-lattice file type utilizing a stable two-dimensional hexagonal closely-packed array of bubbles created in a bubble film and maintained under bias fields leading to states intermediate between stripout and collapse conditions. The bubble diameter and lattice spacing are determined by the applied bias field, the bubble-bubble interactions and magnetic constants of the bubble film.
As desribed in the article, the coding scheme in bubble lattice files makes use of different kinds of domain walls which separate the two states of oppositely directed magnetization inside and outside the bubble. The two simple structures are the "S = 1" state and the "S = 0" state; the latter having opposing senses of magnetization associated with the two halves of the wall separated by vertical boundaries known as "Bloch lines" to distinquish the latter from the S = 1 state which has no "Bloch lines". Since the bubbles associated with both states are magnetostatically identical, digital 1's and 0's are arbitrarily identified as, for example, "1" is S = 1 and "0" is S = 0 of the appropriate lattice sites. The article then goes on to describe a device for detecting the state of reading, writing and accessing the bubbles in the lattice.
In the prior art device described in the Cohen et al article, the bubble lattice is bounded at the top and bottom by horizontal barriers formed by making the bubble film thicker in the region outside the lattice or active area so that magnetostatic forces keep the bubbles within these barriers. Buffer regions on the right and left sides of the lattice area are formed by extra columns of "dummy" bubbles or by horizontal stripped-out domains. Data is accessed by translating the entire lattice horizontally along the rows of the lattice so that various columns of bubbles are brought at will into coincidence with the access channels. These access channels, as well as the buffer regions, are canted in accordance with the symmetry of the hexagonal array. Reading and writing is then carried out by moving the column of bubbles along an access channel to an appropriate write or read station. In order to move the bubbles along the array, pairs of conductors connected in series are spaced periodically along the array and an appropriate bipolar excitation pulses of these pairs generate the perpendicular field gradients.
For reading the data, the state of the bubble is detected by utilizing the fact that in a gradient field S = 0 bubbles travel along the gradient, while S = 1 bubbles move with a velocity component normal to the gradient, thus, two categories of bubbles may be moved in different channels and their presence or absence is sensed magnetoresistively.
The writing operation is performed by pulsing a conductor line so that a local transient in the perpendicular bias field is created. The local bias field transient causes the generation of a S = 1 state in the absence of a substantial in-plain field while an S = 0 state is created in its presence.
Some of the disadvantages and problems with this prior art bubble lattice file device as stated in the above article is that the processing of the bubble lattice file requires many masking levels. This is confirmed in the article by Voegeli, Calhoun, Rosier, and Slonczewski, entitled, "The Use of Bubble Lattices for Information Storage" AIP Conference Proceedings (USA) No. 24, pages 617-19 in 1974 at the 20th Annual Conference on Magnetism and Magnetic Materials, Dec. 3-6, 1974.
One of the reasons for the many masking levels is to provide the thicker bubble material in the region outside the lattice or active area as mentioned above. Also since the prior device is working directly with the bubble material, i.e., not using an overlay pattern, and since the lattices are somewhat difficult to control, it is necessary to groove the material longitudinally to retain the lattices which grooving also requires another masking level.
Another disadvantage in the prior art is the manufacture of the current carrying conductors which makes the device processing tedious and difficult thus reducing the total yield.
In connection with these current carrying conductors, another disadvantage is that since these conductors are directly on the bubble material, the generation of heat by the pulsing of these conductors is a significant problem in this prior art.
Accordingly, it is one object to this invention to provide a bubble lattice file which eliminates the complexities in the manufacture of the prior art devices.
It is still another object to this invention to provide a bubble lattice file which eliminates a number of masks in the process of manufacture of the lattice. Finally, it is a primary object of this invention to provide a bubble lattice file which eliminate altogether the current carrying conductors by utilizing unipolar magnetic fields to propagate and access the memory.
In connection with the foregoing objects and having recognized the disadvantages of the prior art as aforesaid, and turning to the prior art in other bubble memories of the non-lattice type, it should be pointed out that in the U.S. Pat. No. 3,927,398 issued to Magid Y. Dimyan on Dec. 16, 1975 and entitled, "Magnetic Bubble Propagation Circuit" it was shown that a translation force acting on magnetic bubble domains can also be produced by means of a gradient in the spacing between bubble material and the propagate element. As described in this patent an overlay of propagation elements of uniform thickness of material were spaced from a film of bubble material with one end of the element having a greater spacing than the other so as to form a gradient between the bubble material and the ends of each element in the direction of propagation. With this pattern, a periodic monopolar magnetic field applied in the plane of the bubble material and parallel to the propagation path, magnetize the element causing the bubbles to move from one element to an adjacent element across the gap between the element. When the elements were demagnetized, that is, when the magnetic field ceased (being periodic), the bubble moved from the high end of the element to the lower end. Thus, the spacing gradient itself was relied upon to move the bubbles in the absence of an unapplied magnetic field.
This invention takes the teachings of this patent forward and applies the teachings of this invention even though from a non-lattice type memory and utilizes the principle to form the lattice file including the rows as well as the accessing channels or columns thus constituting a considerable improvement both over the teachings of the prior art in connection with bubble lattice files aforesaid.